Construction of a molten CMAS-resistant bilayer-structured apatite layer used for thermal barrier coatings

Molten calcium–magnesium–alumina–silicate (CMAS) is easy to wet and penetrates into thermal barrier coatings (TBCs), causing coating corrosion and premature failure. The application of a protective layer on the TBC surface is considered a useful method to alleviate CMAS attack. In this study, a bilayer-structured apatite layer was developed, on which molten CMAS has low wettability. It had an acicular upper layer and a compact lower layer, which was constructed by pre-reacting GdPO₄ with CMAS powders through precise regulation of the pre-reaction temperature, time, and CMAS concentration. The microstructure integrity of the bilayer-structured apatite layer remained intact after heat treatment at 1250 °C for 50 h, and the layer did not crack after 100 thermal cycles. When exposed to CMAS at 1250 °C for 20 h, the layer retained an intact structure and still functioned to prevent CMAS penetration. At 1250 °C for 30 min, the CMAS contact angle on the bilayer-structured apatite layer was 17.4°, which was three times greater than that on Gd₂Zr₂O_7, which is another promising CMAS-resistant TBC material. Hence, the bilayer-structured apatite layer is quite attractive for addressing the CMAS issue of TBCs.